1. Field of the invention
The present invention relates to a secondary battery, and more particularly to a secondary battery having an improved lead plate securing structure.
2. Description of the Prior Art
Secondary batteries, also known as rechargeable batteries, have been recently researched and developed to have a reduced size and an increased capacity. Representative secondary batteries which have been recently developed and used include a nickel hydrogen battery, a lithium battery, a lithium ion battery, and the like.
In secondary batteries, most bare cells are manufactured in such a manner that an electrode assembly including a positive electrode, a negative electrode and a separator is received in a can made of aluminum or aluminum alloys. A cap assembly is assembled with the can, electrolyte is injected into the can, and the can is sealed. Having a can made of aluminum or aluminum alloys allows a durable lightweight secondary battery to be manufactured such that the secondary battery does not corrode even after long periods of use.
A bare cell is provided with an electrode terminal insulated from the periphery of the bare cell. The electrode terminal is connected to one electrode of the electrode assembly in the bare cell to act as a positive electrode terminal or a negative electrode terminal. The can has a polarity opposite to the electrode terminal.
A battery is an energy source which may discharge a large amount of energy. In the case of the secondary battery, the battery accumulates energy as it is charged. During charging, if an internal short circuit occurs in the secondary battery, the secondary battery may discharge accumulated energy in a short period of time, thereby causing a safety hazard such as a fire or an explosion.
In lithium-based secondary batteries, since lithium is highly reactive, there is a potential for secondary batteries to catch fire and/or explode if the batteries malfunction. In lithium ion batteries, ionized lithium is used rather than metallized lithium, so lithium ion batteries have improved stability as compared to lithium batteries using metallized lithium. However, since the negative electrode or material such as non-hydrogenous electrolyte is flammable, lithium ion batteries still have the potential to catch fire and/or explode if they malfunction.
Therefore, to prevent a secondary battery from catching fire and/or exploding due to charging malfunction, secondary batteries are provided with various safety devices. When the temperature or the voltage of a secondary battery drastically increases due to excessive charging or discharging, the safety devices interrupt electric current to prevent the secondary battery from catching fire and/or exploding. The safety devices are connected to the bare cell, and may include a protection circuit board which detects abnormal electric current or voltage and interrupts the flow of the electric current. The safety devices may also include a positive temperature coefficient element and bimetal, etc. which deform when the secondary battery is overheated due to the abnormal current.
Typically, the electric connection formed by welding the protection circuit board to the electrode of the bare cell is relatively weak because of material characteristics and shape of the bare cell. Thus, a conductive structure, called a lead plate, acts to connect the electric terminal of the safety device to the positive electrode and the negative electrode. Nickel, nickel alloys, or a stainless steel plated with nickel is typically used as material for the lead plate. Generally, the bare cell and the protection circuit board are electrically connected to each other by welding.
FIG. 1 is an exploded perspective view schematically showing a conventional lithium secondary battery before molding resin is attached to the secondary battery. FIG. 2 is an exploded perspective view showing a conventional secondary battery in which a protection circuit board is molded with molding resin and attached to a bare cell. As shown in FIGS. 1 and 2, the can type secondary battery includes a bare cell 10 and a protection circuit board 20. The bare cell 10 is a can 12 which receives an electrode assembly therein and of which an upper end is sealed by a cap plate assembled with a cap assembly.
The cap plate 13 has a size and a shape corresponding to the opened upper end of the can 12. The cap plate 13 includes a cathode contact lead 16 and a lead plate 18 attached to an exterior facing surface of the cap plate. A negative electrode terminal 14 extends through a center portion of the cap plate 13. The cathode contact lead 16 has one end coupled to the negative electrode terminal 14, and the other end extending over the exterior-facing surface of the cap plate 13. The lead plate 18 is attached to the exterior-facing surface of the cap plate 13 such that sidewalls of the lead plate 18 extend away from the can 12.
The protection circuit board 20 has an exterior surface on which external input and output terminals 22, 23 are formed, and an interior surface to which a circuit portion (not shown) and contact terminals 26, 28 are provided. The contact terminals 26, 28 include an anode contact terminal 26 and a cathode contact terminal 28, which are formed in a generally L shape to connect the contact terminals 26, 28 to the lead plates 16, 18, respectively. The lead plates 16, 18 of the cap plate 13 and the contact terminals 26, 28 are generally welded together by resistance spot welding. An insulation plate for electrically insulating the cathode contact lead 16 and the cap plate 13 may be provided.
As shown in FIG. 2, the protection circuit board 20 is molded by a molding portion 30, and then attached to the bare cell 10. The external input and output terminals 22, 23 protrude from a surface of the molding portion 30.
In the conventional secondary battery constructed as described above, the lead plate may become distorted or disengaged from the can 12 when the lead plate is welded to an electrolyte injection hole of the cap plate by a torch. Further, in distortion tests for the can, the lead plate may become deformed due to external force.